LOS ANGELES, NOV. 4 -- Last month's severe southern California earthquake and its several aftershocks were caused by a previously unknown, underground fault that may pass directly under the most heavily populated sections of Los Angeles, according to reports presented today at the first major conference on the quake.

Thomas L. Davis, an independent geologist who is considered an expert on the Los Angeles basin, said his analysis indicates that the Oct. 1 quake originated at the eastern end of a fault marked by a long fold in the earth's crust "extending from West Los Angeles to Whittier."

A series of hills that mark the suspected fault includes the site of Dodger Stadium, a mile north of downtown Los Angeles. The measured movement of tectonic plates in the area, "combined with the presence of concealed faults capable of generating at least moderate-sized earthquakes such as the {last one}, suggest that the long-term earthquake potential of the basin may be underestimated."

The Oct. 1 temblor, now referred to as the Whittier Narrows Earthquake, struck about 12 miles east of downtown Los Angeles and was originally measured at magnitude 6.1. Scientists said today they had reduced its magnitude rating to 5.9, but that the severe damage it caused, and the possibility that its fault is so close to downtown, underlined the importance of further research into its origins.

State officials say damage estimates from the quake and its several aftershocks now total at least $213 million. Authorities have placed the death toll at about six, including three quake-related heart attacks.

Some scientists noted the possibility of a gently, dipping fault in the quake vicinity shortly after the event occurred, but said it might be a twisted extension of the Whittier Fault, a surface fault that ends southeast of the Oct. 1 earthquake's epicenter. Although some scientists argued today that the Whittier Fault could still be the cause, several others expressed the view that they were now dealing with a new, unconnected fault.

"The main shock occurred on a previously unrecognized thrust fault with no obvious surface expression," said seismologists Egill Hauksson of the University of Southern California, site of today's conference, and Lucile M. Jones, of the U.S. Geological Survey, the conference sponsor. "If this fault were to extend farther to the west, it could pose a serious earthquake risk to the city of Los Angeles."